The Black Plague, or The Black Death, was one of the deadliest pandemics in human history. Spread by fleas which lived as parasites on rodents such as rats, mice, and gerbils, it began in Central Asia before sweeping through Europe, devastating the continent and killing a third of the population between 1347 and 1350. In the years after the Plague, the surviving Europeans emerged stronger than before, and were forced to examine themselves spiritually, beginning the path to the Renaissance and Scientific Revolution.
In an alternate known as Agrippan Rome, small plague outbreaks were common, but that world managed to soldier on at the close of the 21st century, without ever having anything on the level of the Black Death occurring.
In one alternate visited by Crosstime Traffic, the Black Plague progressed into the Great Black Deaths, an ongoing series of outbreaks for generations. The Great Black Deaths devastated Europe utterly, killing 80% of the continent's population. As a result, Europe was entirely unable to respond to the continued assaults of Muslim invaders, who captured the majority of southern Europe, leaving an isolated northern Christian Europe alienated by a powerful and dynamic Muslim world.
The scientific advances and theories of the Renaissance and beyond never happened, and inventors such as Johannes Gutenberg were never born. Authors such as Geoffrey Chaucer were killed in the Plague, stunting European culture and art. Europe remained backward and medieval even to the close of the 21st century.
In addition, Catholicism drastically changed, becoming the Church of the New Revelation, a belief system that preached the power of Henri, God's Second Son, who was martyred as a heretic by the Papacy. The subsequent end of the plagues convinced Europeans of Henri's divinity, and a radically different version of Christianity developed in Europe as a result. Jacques was a faithful Christian of the Second Revelation.